White, biaxially oriented polyester films are known from the prior art. These known prior art films are either easy to produce, have good optical properties or have acceptable processing performance.
DE-A 2 353 347 describes a process for producing milky polyester film having one or more layers, which comprises preparing a mixture from particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the mixture as a film, quenching the film and biaxially orienting the film via orientation in directions running perpendicular to one another, and heat-setting the film. A disadvantage of this process is that regrind which arises during production of the film (essentially a mixture of polyester and ethylene or propylene copolymer) cannot be reused without yellowing the film. However, this makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughness of the film is much too high, and this gives the film a very matt appearance (very low gloss), undesirable for many applications.
EP-A 0 300 060 describes a single-layer polyester film which comprises, besides polyethylene terephthalate, from 3 to 40% by weight of a crystalline propylene polymer and from 0.001 to 3% by weight of a surface-active substance. The effect of the surface-active substance is to increase the number of vacuoles in the film and at the same time to reduce their size to the desired extent. This gives the film greater opacity and lower density. A residual disadvantage of the film is that regrind which arises during production of the film (essentially a mixture of polyester and propylene homopolymer) cannot be reused without yellowing the film. However, this makes the film uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughness of the film is much too high, giving it a very matt appearance (very low gloss), undesirable for many applications.
EP-A 0 360 201 describes a polyester film having at least two layers and comprising a base layer with fine vacuoles, with a density of from 0.4 to 1.3 kg/dm3, and having at least one outer layer whose density is above 1.3 kg/dm3. The vacuoles are achieved by adding from 4 to 30% by weight of a crystalline propylene polymer, followed by biaxial stretching of the film. The additional outer layer improves the ease of production of the film (no streaking on the film surface), and the surface tension is increased and the roughness of the laminated surface can be reduced. A residual disadvantage is that regrind arising during production of the film (essentially a mixture of polyester and propylene homopolymer) cannot be reused without yellowing the film. However, this makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughnesses of the films listed in the examples are still too high, giving the films a matt appearance (low gloss), undesirable for many applications.
EP-A 0 795 399 describes a polyester film having at least two layers and comprising a base layer with fine vacuoles, the density of which is from 0.4 to 1.3 kg/dm3, and having at least one outer layer, the density of which is greater than 1.3 kg/dm3. The vacuoles are achieved by adding from 5 to 45% by weight of a thermoplastic polymer to the polyester in the base, followed by biaxial stretching of the film. The thermoplastic polymers used are, inter alia, polypropylene, polyethylene, polymethylpentene, polystyrene or polycarbonate, and the preferred thermoplastic polymer is polypropylene. As a result of adding the outer layer, ease of production of the film is improved (no streaking on the film surface), the surface tension is increased and the roughness of the laminated surface can be matched to prevailing requirements. Further modification of the film in the base layer and/or in the outer layers, using white pigments (generally TiO2) and/or using optical brighteners permits the properties of the film to be matched to the prevailing requirements of the application. A residual disadvantage is that regrind which arises during production of the film (essentially a mixture of polyester and the added polymer) cannot be reused without undefined and highly undesirable changes in the color of the film. This makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the films listed in the examples continue to have excessive roughness values, giving them a matt appearance (low gloss), undesirable for many applications.
DE-A 195 40 277 describes a polyester film having one or more layers and comprising a base layer with fine vacuoles, with a density of from 0.6 to 1.3 kg/dm3, and having planar birefringence of from −0.02 to 0.04. The vacuoles are achieved by adding from 3 to 40% by weight of a thermoplastic resin to the polyester in the base, followed by biaxial stretching of the film. The thermoplastic resins used are, inter alia, polypropylene, polyethylene, polymethylpentene, cyclic olefin polymers, polyacrylic resins, polystyrene or polycarbonate, preferred polymers being polypropylene and polystyrene. By maintaining the stated limits for the birefringence of the film, the film claimed has in particular superior tear strength and superior isotropy properties. However, a residual disadvantage is that regrind arising during production of the film cannot be reused without undefined discoloration of the film arising, and this in turn is highly undesirable. This makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughnesses of the films listed in the examples are still too high, giving them a matt appearance (low gloss), undesirable for many applications.
DE-A 23 46 787 describes a flame-retardant polymer. Besides the polymer itself, its use for producing films and fibers is also described. However, the following shortcomings were apparent during production of films with this phospholane-modified polymer claimed in the DE-A:                The polymer is very sensitive to hydrolysis and has to be very thoroughly predried.        On drying with dryers of the prior art, the polymer coagulates, making it extremely difficult, or even impossible, to produce a film.        The films produced, under conditions which are extreme and not cost-effective, embrittle when exposed to heat, i.e. their mechanical properties deteriorate sharply due to the rapid onset of embrittlement, making the film industrially unusable. This embrittlement arises after as little as 48 hours of exposure to heat.        
The object of the present invention was to provide a white, biaxially oriented polyester film which has high gloss and improved ease of production, i.e. low production costs, and which moreover has high UV resistance combined with good heat resistance and flame retardancy. In particular, it should be possible for cut material (regrind) directly associated with the film production process to be reused in the production process at a concentration of from 10 to 70% by weight, based on the total weight of the film, without any resultant adverse effect on the physical or optical properties of the film produced with regrind. In particular, addition of regrind should not cause any significant yellowing of the film.
High UV resistance means that the film is damaged only slightly or not at all by sunlight or other UV radiation, and therefore that the films are suitable for outdoor applications and/or critical indoor applications. In particular, during outdoor use over a period of some years, the films should not yellow, embrittle or show surface-cracking, nor exhibit any impairment of mechanical properties. High UV resistance therefore means that the film absorbs UV light and does not begin to transmit light until the visible region has been reached.
High flame retardancy means that in what is known as a fire protection test the white film meets the requirements of DIN 4102, Part 2 and in particular the requirements of DIN 4102, Part 1 and can be classified in building materials class B2, in particular B1, for low-flammability materials.
The film should moreover pass the UL 94 test known as the “Vertical burning test for flammability of plastic materials”, therefore qualifying for classification 94 VTM-0. This means that the film ceases to burn within 10 seconds after removal of the bunsen burner, that no further smoldering is observed after 30 seconds, and that no flaming drops are observed over the entire period.
Cost-effective production includes the capability of the polymers or polymer components needed for producing the flame-retardant film to be dried using industrial dryers of the prior art. It is important that the polymers do not cake and do not undergo thermal degradation. These industrial dryers of the prior art include vacuum dryers, fluidized-bed dryers, moving-bed dryers and fixed-bed dryers (power dryers). The dryers mentioned operate at temperatures of from 100 to 170° C., at which flame-retardant polymers usually cake and have to be dug out, making film production impossible.
In vacuum dryers, which have the gentlest drying action, the polymer passes through a range of temperature of from about 30 to 130° C. at a pressure of 50 mbar. A process known as postdrying is then required, in a hopper at temperatures of from 100 to 130° C. and with a residence time of from 3 to 6 hours. Even here, the known polymer cakes to an extreme extent.
Good heat resistance means that the film and its mechanical properties do not deteriorate after 100 hours of annealing at 100° C. in a circulating-air heating cabinet.